In a multibeam scanning optical apparatus including a plurality of light sources for scanning a plurality of lines at the same time, an f-theta (fθ) lens may be designed to have lens surfaces shaped appropriately so that a compact and high-precision image forming apparatus can be realized. Specifically, in designing the shape of f-theta lens, f-numbers may be configured to satisfy the condition: Fmin/Fmax>0.9 where Fmax and Fmin indicate the maximum and minimum values, respectively, of the f-numbers of beams of light incident on a scanned surface in a sub-scanning direction.
In an optical system involving aberrations, unless an adequate consideration is given to the effect of the aberrations, distortions such as curvatures of field would become nonnegligible in view of the loss of image quality. Even if variations in f-numbers could be reduced as described above, variations in resolutions of images formed on the scanned surface could be a possible cause of undesirable unevenness in the resulting image.
Under the circumstances, there is a need to provide an improved scanning optical apparatus which can achieve uniform exposure to light.